Help on SSH authentication for Linux

Hello everyone, for authenticated scan via SSH i tried this passes: start ssh on both machines, create ssh keypair on both machines, upload machine with openVAS’ public key via ssh-copy-id in target machine, and import id_rsa target’s private key via openVAS web interface, in username field i took user of target machine and in passphrase i left blank because of i left blank in configuration with ssh key-gen. In log files i see a lot of authentication failure for some users that i don’t know. How i can improve them?
I talk about a kali machine with openVAS 21 and a ubuntu 22.04 target machine.